Percentages

A percentage is an integer, or a decimal number, followed by a
percentage character (%). Whitespace
characters mustn’t appear between the number and the
%. For example, here’s a
width declaration that uses a percentage value:

#example {
width: 50%;
}

A percentage value is, by its nature, relative to something else. The
interpretation of percentages differs between CSS properties, so you’ll
have to use the reference to find the specific property you’re dealing
with, and to identify what its stated percentage value means. In some
cases, the interpretation of percentage values can be quite unexpected;
for example, vertical padding refers to a percentage of the width—not
height—of the containing block.